In the field of superconducting technology used in a magnetic floating train, a fracture diagnostic apparatus based on nuclear magnetic resonance, or the like, cryogenic technology has been progressed remarkably for various applications including a cryopump used in an ultra-high vacuum apparatus such as a VLSI pattern transfer apparatus. As it has come to the stage of putting the cryogenic technology to practical use, a smaller refrigerator having a higher performance has been progressively developed and realized practically. Especially, there is increasing importance in freezing and cooling techniques for developing an environment having a temperature around the absolute zero temperature (−273° C.) at which the cryopump for creating a high vacuum atmosphere in a superconducting magnet or a semiconductor manufacturing apparatus is operated, and there is a strong demand for a refrigerator exhibiting a high reliability and excellent characteristics.
Conventionally, in a superconducting MRI (nuclear magnetic resonance imaging) apparatus (image measuring apparatus) used for taking tomograms in the medical field, a small refrigerator using helium, such as a Gifford MacMahon type refrigerator (GM refrigerator) has been employed in order to cool and refrigerate the superconducting magnet.
The GM refrigerator is composed of combination of a compressor for compressing He gas, an expansion unit for expanding the compressed He gas, and a regenerator unit (cold accumulating unit) for keeping the He gas cooled in the expansion unit in the cooled state. In the refrigerator, the He gas compressed by the compressor is expanded to be cooled by about 60 cycles per minute, and then, a system to be cooled is cooled through a tip end portion of the expansion unit of the refrigerator.
Alternatively, there have been developed pulse tube refrigerators in recent years. The pulse tube refrigerator is a refrigerator in which high-pressure He gas is provided to the refrigerator in a prescribed period. Since the pulse tube refrigerator has a smaller vibration in comparison with the GM refrigerator, the pulse tube refrigerator has the advantage of suppressing noise generation at measurement in, for example, an MRI apparatus.
In refrigerators used for any purpose, the regenerator unit is filled with a regenerator material (cold accumulating material). As the regenerator material for refrigerator used in an extremely low temperature region around the absolute zero temperature, for example, of 10K or less, further of 4K or less, a rare-earth regenerator material disclosed in Japanese Patent No. 2609747 (Patent Document 1) is technically effective. The technique disclosed in the Patent Document 1 involves adjusting the particle size and the aspect ratio of rare-earth regenerator material particles, and thereby enables high-density filling of the regenerator material particles.
On the other hand, studies for employing multi-staged regenerator units have been made in view of improving performance of the refrigerator. For example, Japanese Patent Laid-Open No. 2001-272126 (Patent Document 2) discloses a two-staged pulse tube refrigerator. By employing multi-staged regenerator units, it is possible to attain a higher cooling velocity (refrigerating speed). It is also possible to attain a larger cooling amount, thereby the refrigerator can be installed in a larger-scaled apparatus. By employing the multi-staged regenerator units, He gas must be supplied in a larger amount at a higher pressure.
The refrigerating performance of a refrigerator depends on amount of He gas having contact with the surface of the regenerator material particles. As the regenerator material particles, particles having a spherical shape have been conventionally used in view of realizing high-density filling, and therefore, it is difficult to fill the particles at a higher density than ever before. Although there is a conceivable measure of filling the space between the regenerator material particles with smaller regenerator material particles, filling particles in an excessively large amount leads to a decrease in permeability of He gas serving as a cooling medium. Another measure of filling the particles under a high pressure is also conceivable; however, filling under an excessively high pressure leads to crushing of the regenerator material particles, which rather causes clogging in a cooling system. Therefore, there is a technical demand for a regenerator material capable of ensuring the high-density filling and attaining a larger contact surface area with the He gas.